Timo and I agreed previously that we should ditch the class pattern for view models and instead have them be interfaces which are simply created by functions. They're more straightforward to write, mock, and instantiate this way.
The code for media view models and media items is pretty much the last remaining instance of the class pattern. Since I was about to introduce a new media view model for ringing, I wanted to get this refactor out of the way first rather than add to the technical debt.
This refactor also makes things a little easier for https://github.com/element-hq/element-call/pull/3747 by extracting volume controls into their own module.
